Methods of producing fire resisting wooden articles



United States Patent 3,498,877 METHODS OF PRODUCING FIRE RESISTING WOODEN ARTICLES Christen Christotfersen and Karl-Otto Sorensen, Nestved, Denmark, assignors to Vedex Dansk Skovindustrre A/ S, Nestved, Denmark, a company of Denmark No Drawing. Filed Mar. 1, 1965, Ser. No. 436,355 Claims priority, application Denmark, Mar. 4, 1964, 1,069/64; Sept. 28, 1964, 4,762/64 Int. Cl. B32b 13/10, 27/42; C09j /02 US. Cl. 161-209 18 Claims ABSTRACT OF THE DISCLOSURE Fire resisting wooden building articles are prepared by adhering thin pieces of wood under heat and compression with an adhesive comprising synthetic resin, koalin and diammonium phosphate, the pieces of Wood having been previously impregnated with diammonium phosphate. The building articles may be in a variety of forms, such as boards or panels, the latter being based on veneers prepared from single pieces of Wood or from plural pieces of Wood, e.g. chips or shavings.

The present invention relates to a method of producing a fire resisting wooden building article having particularly high fire resisting properties compared with hitherto known fire resisting wooden articles.

An object of the invention is to provide a method of producing fire resisting wooden building articles, such as boards or panels, based on veneer and capable of resisting high temperatures for a long period without burning through and without through holes being formed in the board when it is burned through and its rear side begins to char.

We have discovered that an improved fire resisting wooden construction board or the like can be prepared when pieces of veneer, such as shavings or veneer plates, are dried, then impregnated with diammonium phosphate ((NH HPO and thereupon glued together under heating and pressing by means of an adhesive comprising a synethic resin, koalin and diammonium phosphate.

It has been found that, during the hardening of such an adhesive, the resin will not only harden but will also react with the diammonium phosphate. The resistance of each piece of veneer against heat and flame is thus substantially increased so that any charring of the veneer takes a long time; after charring, the veneer remains united without peeling apart. The great heat insulating property in coke further contributes to a delay of the burning through and consequently to an increase in the fire resistance. Moreover, the kaolin in the adhesive has a heat distributing effect whereby local superheating Within the connections between the veneer pieces and thus limited local burning through of the veneer pieces is avoided.

The result is that, for example, a 25 mm.-thick plywood board produced by the method of the present invention and at temperatures and pressures normal for the production of plywood boards, i.e. temperatures of between 70 and 150 C. and a pressure of 4-30 kg./m. is capable of resisting the aforesaid flame influence for at least 60 minutes, after which time the rear side of the plate may start charring, but without through holes appearing in the wood. On the front side of the board exposed to the flame only the part directly hit by the flame becomes charred the board offers a good resistance to the spreading of a fire.

A further increase of the fire resisting properties may be obtained if, in addition to the diammonium phosphate, the veneer pieces are impregnated with copper sulphate (CuSO,) which has proved to have a heat distributing effect within veneers, whereby the danger of local super heating is further reduced. Said addition of copper sulphate further causes the board to become resistant against rot, fungus, and vermin attacks.

When copper sulphate is added to the veneer, the latter becomes dark colored which, however, in many cases can also be considered an advantage.

The content of kaolin and diammonium phosphate in the adhesive may vary within rather wide limits, inter alia depending on the type of synthetic resin used and the solvent or emulgation agent employed for the adhesive. If, as most often is the case, an aqueous adhesive solution is used, it has been found that the best results are obtained when the adhesive solution has a content of 30 to 50% synthetic resin, 20 to 50% kaolin and 0.5 to 2% diammonium phosphate. Preferably a solution is used having a content of about 40% synthetic resin, 1% diammonium phosphate and 25% kaolin.

Also the synthetic resin may be of different types. Thus good results have been obtained by using urea formaldehyde together with melamine powder, for example in a proportion of 4:1 by weight, phenolformaldehyde with or without the addition of melamine powder, melamine formaldehyde and epoxy-adhesives.

In order to obtain the best possible fire resistance, it is of importance that the veneers are evenly impregnated with the impregnating agent employed, that is diarnmonium phosphate alone or together with copper sulphate or other substances, and so that at least 30%, preferably about 100% of the pore cavities of the veneer are filled with salts. Some effect is obtained, however, already at a degree of filling of 15%.

For obtaining this, it may be advantageous to impregnate the veneer pieces by first drying said veneer pieces almost completely in a drying oven, and then placing them, in hot condition, in a vacuum tank, which is evacuated, whereupon an aqueous solution of the impregnation agent is filled into the tank, whereafter the veneer pieces are dried anew after removal from the vacuum tank.

In consequence of the first drying process practically all pores of the veneer will become evacuated during the evacuation of the container, so that the impregnating solution will be able to penetrate into all pores and fill the same. At the subsequent drying, which is preferably effected to such an extent that the finally dried veneer has a moisture content of about 6%, the impregnating agent is deposited in all the pores and is thus evenly distributed throughout the veneer.

Preferably an aqueous diammonium phosphate solution with a content of 15 to 40 percent by weight of diammonium phosphate is used. If copper sulphate is used too, the solution used for the impregnation may have a content of between 0.5 and 3% copper sulphate. If the impregnating solution contains less than 0.5% copper sulphate, there will be no improvement of the fire resistance, and an increase in the content of copper sulphate beyond 3% will cause no further increase in the fire resistance, and, therefore, it will be bad economy to use larger quantities.

Even if, as mentioned above, a plywood board or panel produced by the method of the present invention has particularly good fire resisting properties, it is possible with the method of the invention to increase the fire resistance still more, if, during the gluing process, the veneer pieces are exposed to such a pressure that each piece of veneer is compressed sufficiently to decrease its thickness from 10 to 50%, preferably 25%. In this manner it is possible to obtain a compact board or panel which not only has a particularly great strength, as Well as good nail and screw holding properties, resistance to moisture and direct influence from water as well as to boiling, but which further has quite extraordinarily high fire resisting properties compared to known plywood boards or panels. Thus, a plywood board produced under such a pressure by the method of the invention and having a thickness of 25 mm. will, when exposed to a flame be able to resist the flame for more than 150 minutes before the rear side of the board begins charring until shortly before such charring the temperature will be at most 160 C. At the same time the charring of the front side of the board will be distinctly limited to the zone directly hit by the flame.

It should be noted that, irrespective of the degree of compression of the board, the latter will not, when exposed to the flames, be able to burn itself, not even on the surface, nor will any smoke be developed. This latter quality is of great importance when the plate is to be used as a fire resisting building material.

An increase of the fire resisting properties of the board due to high compression of the veneers is obtained by use of a pressure as low as 100 kg./cm. but preferably depending on the species of the wood pressures above 250 kg./cm. ought to be used.

Example 1 Twenty-one equally large beech veneer plates of a thickness of 1 mm. are dried in a drying oven to a moisture content of 4%, Whereafter the veneers are, in hot condition, placed in a vacuum tank, which is pumped almost empty of air. Thereafter a 30% aqueous solution of diammonium phosphate is let into the tank, and after some time the tank is drained and then opened, and the veneer plates are taken out and again placed in the drying oven, where they are dried to a moisture content of about 610%.

An aqueous adhesive solution is prepared, consisting of 125 parts by weight of phenol-formaldehyde, 3 parts by weight of diammonium phosphate, 75 parts by weight of kaolin and 97 parts by weight of water. The dried impregnated veneer plates are placed on top of each other, every second plate being provided with a 0.2 mm. thick layer of the said adhesive solution on both its sides, whereafter the veneer pile is placed in a press heated to 125 C., and is compressed under a pressure of about 30 kg./cm. for the production of a 25 mm.-thick plywood panel. The mechanical properties and resistance of the latter against moisture are the same as the corresponding properties of an ordinary plywood board in which the veneers are glued together by means of phenol-formaldehyde, but contrary to what is the case with such a board, the panel produced according to the example cannot burn and will not, when heated, be able to develop inflammable gases. If, however, its one side is exposed to the flame from a butane gas blow burner, it will become charred, but the said charring will penetrate so slowly through the plate that complete charring will not take place until after a flame influence for a time of 60 minutes or more. Even after such complete charring, the plate will remain intact and not show through holes, just as there will be no peeling off from the charred places.

Example 2 Forty-five 1 mm.-thick veneer plates are impregnated with diammonium phosphate in the manner stated in Example I, however, by the use of a 25% aqueous solution of diammonium phosphate, and are then dried to a moisture content of about 610% An adhesive is prepared by mixing 100 parts by weight of ureaformaldehyde, parts by weight of melamine powder, 3 parts by weight of diammonium phosphate, 75 parts by weight of kaolin and 97 parts by weight of water.

The forty-five veneers are piled on top of each other, a 0.2 mm. thick layer of said adhesive being applied to every second veneer on both sides thereof, whereafter the pile formed is placed in a press. The latter is heated to a temperature of between 110 and 125 C., for example 115 0., and is caused to exert a pressure on said pile of about 300 kg./cm. which compresses the veneers to such an extent that their thickness decreases up to 50%. The veneer pile is kept in the press under these conditions for a period of 1620 minutes, and the result is a plywood board of a thickness of 25 mm.

This board has quite extraordinary mechanical properties both as regards strength and nail and screw holding properties. Moreover, said plate is completely resistant to any form of moisture influence, and to moist air at relatively high temperatures as may be found in tropical countries.

In addition to said particularly high mechanical properties the board or panel will, however, also have a fire resistance that far exceeds what has hitherto been obtainable even with the use of fireproof materials in the production of fire resisting wooden boards. Thus such a board will, when influenced by a butane gas blow burner, only show a beginning charring on its rear side after a constant flame influence over a period of more than 150 minutes. Even at that time, the board will remain completely intact showing no peeling and only showing a comparatively slight bulging. Even on the side of the board which is directly exposed to the flame, the charred part will be distinctly limited to the Zone directly hit by the flame, and the charring will have taken place without any appreciable development of smoke.

Example 3 Following the procedure of Example 2, fifteen veneers are used to prepare a compressed plywood board of a thickness of 10 mm. The latter has the same mechanical properties as the board produced according to Example 2, but has, on account of its smaller thickness, a lower fire resistance. The 10 mm. thick board will, however, be able to resist the said flame influence for 50-60 minutes.

Example 4 Thirty-four equally large plate-shaped veneers of a thickness of 1.4 mm. were dried in a drying oven to a moisture content of about 5%, whereafter the veneers were, in hot condition, placed in two vacuum tanks, seventeen in each tank. The tanks were pumped almost empty of air, whereafter an aqueous solution was introduced into the first tank containing 20% diammonium phosphate and 0.5% polyethylene glycol, while an aqueous impregnation solution of 20% diammonium phosphate, 1.5% copper sulphate and 0.5% polyethylene glycol was let into the second tank. After some time the two tanks were drained and opened, and the two sets of differently impregnated veneers were taken out and again placed in the drying oven, where they were dried to a moisture content of about 6%.

An adhesive was prepared by mixing parts by weight of phenolformaldehyde, 10 parts by weight of para-formaldehyde, 5 parts by weight of diammoniumphosphate, 50 parts by weight of kaolin and parts by weight of water. The seventeen veneers of each set were piled on top of each other, every second plate having applied on each side thereof a layer of adhesive about 0.2 mm. thick Whereafter the two piles thus formed were placed in a press. The latter was heated to a temperature of about C. and actuated on the piles with a pres sure of about 100 kg./cm. The veneers remained under these conditions in the press for a period of 20 minutes, and two plywood boards were formed each having a thickness of about 22 mm.

Each of said two boards was, at its center, exposed to the flame of a butane gas burner under exactly the same conditions. It was found that the board that had only been impregnated by diammonium phosphate was able to resist the flame influence between 1% and 1% hours before any charring appeared on the rear side of the board, whereas the board, the veneers of which were treated with copper sulphate in addition to diammonium phosphate, did not show any charring on the rear side until after 1% hours.

In the aforesaid examples the method according to the invention has been explained in connection with plywood boards or panels based on plate-shaped veneers, that is in the same manner as is used in the production of plywood and corresponding boards or panels. This is due to the fact that the method according to the invention has a special advantage in the production of fire resisting wooden boards with excellent fire resistance in the form of such plywood boards or panels composed of piled veneers which yield a cheaper board than would be possible were the board composed of shavings or corresponding smaller pieces of veneer. Naturally, however, the new method may also be used for the production of fire resisting boards or panels made from shavings or similar small pieces of veneer and will, in connection with such boards, give the same fire retarding effect as has been explained above.

We claim:

1. A method of producing a fire resisting wooden building article from thin pieces of wood, comprising drying the thin pieces of wood, impregnating said pieces with diammonium phosphate ((NH HPO and gluing said pieces together under heat and pressure with an adhesive comprising synthetic resin, kaolin and diammonium phosphate.

2. A method according to claim 1 wherein the adhesive is an aqueous solution comprising 30 to 50% synthetic resin, 20 to 50% kaolin and 0.5 to 2% diammonium phosphate.

3. A method according to claim 1 wherein the adhesive comprises, in addition to the synthetic resin, about 1% diammonium phosphate and 25% kaolin.

4. A method according to claim 1 wherein the synthetic resin comprises a mixture of a ureaformaldehyde resin and melamine powder.

5. A method according to claim 4 wherein the weight ratio of said ureaforrnaldehyde resin to said melamine powder is about 4: 1.

6. A method according to claim 1 wherein the synthetic resin comprises a phenolformaldehyde resin.

7. A method according to claim 6 wherein the synthetic resin comprises melamine powder.

8. A method according to claim 1 wherein the syn thetic resin comprises a melamineformaldehyde resin.

9. A method according to claim 1 wherein the synthetic resin comprises an epoxy adhesive.

10. A method according to claim 1 further comprising impregnating said pieces of wood with copper sulphate (CuSO proir to gluing said pieces together.

11. A method according to claim 1 comprising sequentially drying said pieces of wood in a drying oven to a moisture content below 10%, placing said pieces, while still hot, in a vacuum tank, evacuating said vacuum tank, charging said tank with an impregnating liquid comprising diammonium phosphate, removing said pieces from said tank when the former are impregnated, drying the impregnated pieces in a drying oven to a moisture content below 10% and gluing said pieces under heat and pressure with said adhesive.

12. A method according to claim 11 wherein the impregnating liquid includes copper sulphate (CuSO for distributing the heat among the pieces of wood in the wooden building article and for reducing the danger of local superheating therein when said article is exposed to flame.

13. A method according to claim 1 wherein the pressure is suificient to compress each of said pieces of wood so that its thickness decreases between 10 and 50%.

14. A method according to claim 13 wherein said pressure is sufficient to compress each of said pieces of woo so that its thickness is decreased about 20%.

15. A method according to claim 1 wherein the thin pieces of wood are plates of veneer and the pressure is in excess of kg./cm.

16. A method according to claim 15 wherein the pressure is in excess of 250 kg./cm.

17. A wooden board comprising plural diammoniumphosphate-impregnated thin pieces of wood adhered to gether by a synthetic resin adhesive comprising kaolin and diammonium phosphate.

18. A plywood board according to claim 17.

References Cited UNITED STATES PATENTS 1,766,606 6/1930 Coolidge 117-138 1,804,633 5/1931 McKee 117-438 1,994,073 3/1935 Hartman et al. 117138 X 2,140,981 12/1938 Booty et a1 11759 2,452,054 10/1948 Jones et al. 2528.1 X 2,532,983 12/1950 Alderson 161209 2,628,946 2/1953 Juda et a1 161191 X 3,284,216 11/1966 Kaplan 252-81 X JOHN T. GOOLKASIAN, Primary Examiner D. J. FRITSCH, Assistant Examiner US. Cl. X.R. 

